The battery performance of rechargeable lithium batteries may depend on the characteristics of electrodes used. Battery cells have been made by bonding tabs to current collectors, such that the tabs are capable of conveying current from a current collector. Some examples of such cells may include a rectangular-shaped cell in which a plurality of rectangular-shaped electrodes are laminated or stacked; or a cylindrical-shaped cell in which band-shaped electrodes are spirally-wound. For instance, a spirally-wound lithium battery may include a positive electrode made by coating a band-shaped collector foil with a positive electrode active material, a plurality of tabs bonded to a part of the positive electrode, a separator, and a negative electrode made by coating a band-shaped negative collector foil with a negative electrode active material and a plurality of tabs superimposed on the positive electrode by way of the separator and a second separator imposed on the top of the assembly. These components may be wound integrally in a spiral manner. Tabs may be bonded to an electrode by methods such as ultrasonic welding, resistance welding, laser welding, stamping, or crimping.
Batteries have included tabs made by using a metallic foil, which has been attached to each of the positive and negative electrodes. A current could flow in each of the tabs when the small-sized battery is discharged or recharged. A tab can be attached at a leading end of a wound band-shaped electrode, at a trailing end thereof, or at a point between the leading end and trailing end thereof.
A band-shaped electrode may be formed as a long and continuous substance and has a large surface area. In such cases, sometimes a plurality of tabs have been attached over the electrodes at intervals so that the discharge and recharge take place somewhat uniformly over the entire length of the electrodes. In some cases, tabs have been aligned in the spiral, although the tightening at the center of the winding makes it difficult to align them at some portions.
A resistance exists within an electrode along the length of the electrode. If the density of a current perpendicular to the electrode were not uniform over the electrode surface, the utilization of the high current density area would be complete before that of the low current area. This may cause the effective area of the electrode to steadily decrease and impedance to rise, such that the current capability of the electrode may be diminished.
If a current collector foil is too thin, the current collector resistance along the electrode may be high. If a current collector foil is too thick, the electrode area may be small and the electrochemical resistance may be high. Increasing either current collector resistance or electrochemical resistance may cause the current capability of an electrode to diminish, and the performance of a battery to suffer.
Therefore, a need exists for a configuration of tabs and optimizing current collector thicknesses such that the current collector configurations may lead to reductions of current collector impedance while maintaining maximum uniformity of current density of the electrodes and equalizing resistance between the electrodes in order to increase the performance of the battery.